asterisk users - Sep 2008 - Combine sip audio and video from different sources

Hello list,

Is it possibe to "multiplex" audio and video from 2 different sip
phones and
transmit both during one single call?

I would like to setup something similar to the Cisco Video Advantage.

Lets asume the following setup:

Both parties have a sip hardphone and a sip video softphone.
Both, the hardphone and the softphone register them self to asterisk.

If one party calls the other party then the hardphone should ring.
If the other side picks up the phone, then video should be transmitted
between the soft-phones and audio between the hard-phones.

(I think this can be achieved with some dailplan rules, setting up
the softphone to auto-pickup and disabeling the audio codecs on the
video phone (we just establish 2 seperate calls one for video and one
for audio)).

But the more difficult thing is what to do if one side has just a
sip-video phone without a stand alone hardphone. Now audio and video
from the hard and softphone must be "multiplexed" and transmitted to
the
single sip-video-softphone, so the other end can talk and watch using
just the soft phone.

Is something like this possible? Any ideas?

Thanks, Artem